3.1. Microscopic Observation
The color of the patinas on the surfaces of the 16 bronze sculptures that had been exposed to outdoor environments for over 20 years were all shades of green and classified into three main groups (Fig. 2a).
The type I dark green patina layers (sculptures SC1, SC3, SC4, SC6, SC8, SC9, S11, SC12, SC14, SC15) were observed to have mostly flat and dense surfaces. The type Ⅱ patina layers were light green (sculptures SC5, SC7, SC10, SC13, SC16), and the type Ⅲ patina layer was close to a light blue patina (sculpture SC2). Specifically, the sky-blue and light green patina layers were less dense on the surface than the dark green patina layers and exhibited some pores and damaged areas.
A comparison between the surface conditions of the outdoor bronze sculptures’ natural and artificial patinas revealed differences in density and other characteristics. The reason for these differences was determined to be the weak adhesion and porosity between the base layer and artificial patina generated over a short period of time. Furthermore, the outdoor bronze sculptures had been exposed to an acidic environment with an average pH of 4.8 and atmospheric pollutants such as SO2 and NO2 for over 20 years, as confirmed by the current levels of air pollution and acid depositions in the area. The corrosion type and patterns were anticipated to be similar owing to the similar alloy compositions and close proximity within a 300-m radius; however, differences in the color and surface conditions of the patinas were observed. This is likely owing to various parameters such as the surface condition at the time of production, solubility of the corrosion products, presence of protective measures (e.g., surrounding vegetation), different corrosion conditions (e.g., bird droppings, pollen, and pine pollen), preservation treatments (wax coating), and impurities within the alloy (segregation).
3.2. Chromaticity and Reflectivity Analysis
The chromaticity and reflectivity analyses revealed that the patinas on the surfaces of the 16 bronze sculptures were all in the green color range and could be classified into three main groups. The change in the b* value was relatively small compared to the significant changes observed in the L* and a* values. Based on the analyses of actual sculptures, the chromaticity and reflectivity as well as the applicability of sulfide and chloride artificial patinas were evaluated through artificial patina corrosion experiments (Figs. 3 and 4).
The sulfide artificial patina closely resembled the surface patina color of bronze sculptures exposed to outdoor corrosion (Fig. 3). The L* value gradually increased from 53.50 to 58.86, indicating a brightening effect. The analysis of the b* values representing the yellow–blue color showed no significant changes after the initial stage (S5), but gradually shifted toward the blue color. Specifically, the analysis of the a* values representing the red–green color revealed a color shift similar to that of outdoor bronze sculptures (-8 to 0), and the L* values were also within the brightness range of the bronze sculptures. The correlation between the growth of patina and chromaticity of the outdoor bronze sculptures was confirmed to progress from type I to type II to type III by the chromaticity and corrosion behaviors of the artificial sulfide patina.
The chloride artificial patina showed a significant difference in L* values of 20–40 compared to the artwork, with a* and b* values concentrated in the ranges of -18 to -15 and − 3.6 to 6.4, respectively, indicating differences from the actual patina colors of the sculptures.
The reflectivity analysis yielded similar results (Fig. 4). The reflectivity of the outdoor bronze sculpture revealed low reflectivity across all wavelength ranges, gradually increasing in the blue (380–500 nm) and green (500–565 nm) spectra. However, a decrease in reflectivity was observed as the transition was made from the yellow (565–625 nm) to the red (625–750 nm) spectrum. The patina of alkaline sulfide also exhibited a similar spectrum to that of the sculpture. The blue–green spectrum increased, whereas the yellow–red spectrum decreased consistent with the corrosive behavior. The artificial chloride patina exhibited higher reflectivity than the artificial sulfide patina in all wavelength ranges for the outdoor bronze sculptures. Particularly, a reflectivity difference of 40–50% or higher was observed in the blue–green spectrum.
The above findings confirm the possibility of identifying corrosion type for bronze sculptures using the chromaticity and reflectivity of artificial patinas. Chromaticity was found to be particularly suitable for the quantitative comparison with other colors, while reflectivity was useful for identifying the corrosion type of specific colors. However, the wide distribution values of brightness and reflectivity of outdoor bronze sculptures also showed discrepancies (L* was approximately 12–17) for the artificial sulfide patina. The differences in surface conditions of natural and artificial patinas on outdoor bronze sculptures are influenced by factors such as dust, pollen, and atmospheric pollutants, which affect the brightness and reflectivity of the sculptures. In addition, the optical characteristics of the patina surface can be altered by light diffraction and absorption, as well as interference effects such as the patina layer thickness, refractive index, and incident light.
One of the crucial factors in distinguishing bronze patinas is known to be the L* value [10]. According to the optical properties of materials, an object's reflectivity generally influences its color and brightness. When an object exhibits higher reflectivity across all wavelengths compared to the reference, it appears brighter [40]. The artificial chloride patina exhibited a brightness (L*) value more than 20 higher than that of the outdoor bronze sculpture and artificial sulfide patina, with high reflectivity observed across all wavelength ranges. In particular, a difference of 40–50% or higher in reflectance was observed in the blue–green spectrum. This suggests that brightness (L*) can be used in conjunction with reflectivity to identify the type of corrosion of a bronze patina.
3.3. Portable XRF Analysis
The chemical compositions of the patina surfaces of the 16 bronze sculptures were classified into three main groups. Based on the analysis of actual sculptures, the results of the corrosion experiment components of sulfide and chloride artificial patinas (Fig. 5) were compared, and the applicability was assessed using a Zn-Sn-Pb ternary diagram (Fig. 6).
The artificial sulfide patina exhibited a tendency similar to the alloy composition of bronze sculpture type I. When compared and analyzed based on bronze sculpture type I, the standard deviations of the elements were found to be 2 wt% for Cu, -1 wt% for Zn, -0.5 wt% for Sn, and − 0.5 wt% for Pb. We attempted to observe changes in the alloy elements resulting from artificial patina corrosion behavior in a ternary system; however, no significant changes were observed in the ternary diagram. This suggests that significant compositional changes are unlikely in a short period of time (2,400 h).
The chemical compositions of artificial chloride patinas were broadly classified into three groups: type A (Cl1–Cl3) for early corrosion, type B (Cl4–Cl25) for intermediate corrosion, and type C (Cl26–Cl50) where Zn was not detected. Specifically, type A was distributed along the boundary of the bronze sculptures type I and type II, whereas the other groups exhibited differed significantly from the sculpture composition.
The chemical composition of the outdoor bronze sculptures showed an overall decrease in Cu and Zn contents, whereas the Sn content exhibited an increasing trend. However, a consistent trend was difficult to demonstrate owing to factors such as alloy composition, corrosion degree, surface condition, and various environmental parameters. Notwithstanding, Cu exhibited distinct differences in composition from the other elements according to the corrosion behaviors of the sulfide patina (87.47→89.04 wt%) and chloride patina (88.8→78.2 wt%). Hence, the compositional differences according to the corrosion behavior of Cu are likely highly applicable in identifying the corrosion type of sulfide and chloride patinas.
The identification and quantification of patina layers are known to be achievable in the range of 20–100 µm using p-XRF, depending on factors such as the alloy of bronze and the degree of corrosion [11, 14]. In particular, owing to environmental influences, the patina layer on bronze artifacts exposed to outdoor conditions is expected to be thin and less complex in terms of corrosion mechanisms than artifacts with thick patina layers. Therefore, portable XRF analysis is expected to be applicable.
3.4. Portable Raman Spectroscopy
Before analyzing the corrosion products on the patina surfaces of the outdoor bronze sculptures in the field, previous studies have already conducted preliminary analyses on the corrosion products of sulfide and chloride artificial patinas [30, 31]. The potential applicability of portable Raman spectroscopy was confirmed by comparing the analysis results of the corrosion products on the surfaces with artificial patinas using XRD, bench-top Raman microspectroscopy, and portable Raman spectroscopy.
An analysis of the portable Raman spectroscopy on the patina surfaces of the 16 outdoor bronze sculptures revealed the presence of cuprite, brochantite, and antlerite (Fig. 7). Raman shifts at 214, 216, and 218 cm-1 were detected in eight sculptures (SC1, SC4, SC7, SC8, SC10, SC12, SC13, SC14), closely matching those of cuprite (Cu2O). The similarity in Raman shifts and peak shapes confirmed the presence of cuprite.
Raman shifts of 384, 422, 480, 970, 971, and 973 cm-1 were detected in 12 sculptures (SC1, SC2, SC4, SC6, SC7, SC9, SC10, SC11, SC12, SC14, SC15, SC16). These were confirmed to be brochantite (Cu4SO4(OH)6), as their Raman shift closely matched that of alkaline sulfide, and their peak shapes were also similar. In particular, cuprite and brochantite were detected together in 6 sculptures (SC1, SC4, SC7, SC10, SC12, SC14).
In two sculptures (SC11, SC15), Raman shifts of 420, 971, 973, and 1047 cm-1 were detected, closely matching the Raman shift of the basic sulfide antlerite (Cu3SO4(OH)4). The similarity in peak shape further confirmed the presence of antlerite.
Additional verification and research are deemed necessary for two sculptures, SC3 and SC5, owing to difficulty in accurately identifying the compounds present in these sculptures.
These results are consistent with previous visual and microscopic observations, chromaticity and reflectivity, and portable XRF analysis results. Thus, we confirmed that the identification of corrosion type and the corrosion products of a bronze sculpture is possible using portable Raman spectroscopy.
When bronze is exposed for decades to the outdoor environment contaminated with SO2, the SO2 dissolves in the surface moisture and reacts with the bronze to form alkaline sulfides. The most commonly observed alkaline sulfide in outdoor bronze sculptures is green brochantite. Brochantite is typically stable under acidic conditions (pH 3.5–6), while antlerite is commonly found in regions with low acidity (pH 3.5 or lower) and high concentrations of sulfates.
In actual outdoor environments, acidity from rain, snow, and fog can dissolve copper corrosion products on the outermost layer and promote the alteration of minerals. Two examples are brochantite and antlerite. When the acidity of the water membrane where corrosion occurs increases (pH 3.5 or lower), brochantite dissolves and antlerite is formed. However, the formation of antlerite requires the prolonged maintenance of acidic conditions in the water membrane layer over a significant period. A short duration of acidic conditions can prevent the growth of antlerite and allow the surface recrystallization of brochantite. Thus, antlerite has been reported to form first in the lower parts or relatively less exposed areas of outdoor bronze sculptures where the moisture evaporation rate is lower [21, 25, 41]. Therefore, the presence of antlerite indicates a higher level of contamination, requiring increased caution regarding potential damage.